Processed foods have become a scapegoat for many of the health problems affecting Western societies. These ready-to-eat items typically found in cans, plastic packaging, and frozen aluminum sheets contain high levels of salt, fat, and readily digestible carbohydrates. These unhealthy features are a result of both consumer preference (they make the foods taste good) and factory processing (they are amenable to industrial food production). And there are compelling links between high levels of salt, fat, and sugar and serious health conditions like hypertension and type 2 diabetes.
One detail missing from the description of processed foods pertains to what they lack: Fiber, including the fermentable prebiotic fiber that feeds the gut microbiome. Bleached white flour is the primary ingredient in many staples of the Western Diet and the processing involved in turning wheat into white flour strips away much of the fiber. For example, choosing all purpose white flour instead of whole wheat flour results in a loss of approximately 80% of the fiber content.
That said, not all processed foods are inherently bad for you. Consumer awareness has created demand for processed foods that provide convenience and shelf stability in combination with a good nutrition profile. Examples of these types of foods include no-salt-added vegetables and beans, as well as no-sugar-added corn when packaged in BPA-free cans. Similar options exist for frozen vegetables pre-cut and mixed for applications like stir fries and pasta dishes. In the case of fermented dairy products, processing involves live microorganisms that consume lactose, which actually helps to lower the amount of available carbohydrates in these foods.
One type of processing that many people forget about is the ripening of fruits and vegetables. This involves the enzymatic conversion of fibrous carbohydrates into sugars like glucose and fructose, increasing the digestibility of these foods. An excellent example are bananas, which contain resistant starch (a type of prebiotic fiber) while unripe but then convert that starch into readily digestible forms after ripening. This change is apparent in both the transition from green to yellow and from a bland to sweet taste.
While many prebiotics like inulin are generally immune to the effects of processing, others like resistant starch are not. And given that different prebiotics have different health properties, one cannot consume their daily fiber intake from inulin-rich foods alone and expect to have a healthy microbiome. Pressure and heat are particularly degrading to resistant starches, meaning that most foods that undergo normal kitchen preparations run the risk of losing their resistant starch content. And because many of the forms of resistant starch that our ancestor would have consumed unripe are now shipped to us in near-perfect ripeness, it can be very difficult to get enough resistant starch from diet alone.
Processed foods don’t need to be completely avoided if you know what to look for. But remember that not all of the processing happens at the factory: Cooking starchy foods and allowing fruit to ripen are also forms of processing that we accept as part of a healthy diet. Resistant starch is an elusive prebiotic, so pay close attention to your diet when trying to increase levels of this ‘healthy’ starch.